Each Territorial Army unit had a small number of regular NCOs attached as instructors, and as specialist officers such as adjutant or quartermaster. They were posted for two or three years before being returned to duty with their regular corps.
This was certainly the case post-1946, and I believe it was the same in the pre-and post-WW1 days.
Your man may not have retired from the regular army, but did not serve in a regular unit, while attached as a Permanent Staff Instructor.
A